Harnessing fundamental physics of topological insulators for thermoelectric energy conversion

June 19, 2014 - In the recent Nano Letters article, Prof. Nikolic and DPA graduate student Po-Hao Chang, in collaboration with Prof. Naoto Nagaosa and Prof. Mohammad Saeed Bahramy from the University of Tokyo and RIKEN Center for Emergent Matter Science in Japan, have proposed a new type of nanoscale thermoelectric which exploits fundamental physics of topological states of matter to convert waste heat into electricity.   

Thermoelectrics transform temperature gradients into electric voltage and vice versa. Although a plethora of thermoelectric energy harvesting and cooling applications has been envisioned, their usage is presently limited by their poor efficiency. This is due to the fact that efficiency measured by the figure of merit ZT requires carefull tradeoff to increase electrical conductance G and the so-called Seebeck coefficient S, while decreasing  the thermal conductance Ktot due to electrons and lattice vibrations. The values of ZT approaching infinity would ensure Carnot efficiency as the theoretical limit for a heat engine operating between a hot and a cold temperature. However, ZT of realistic devices is limited by irreversible energy losses via Joule heat and thermal conduction, so that a pragmatic goal is to achieve ZT around 3. 

The recently discovered topological insulator (TI) materials are of particular interest for thermoelectricity. The key ingredient in this new class of materials is strong spin-orbit coupling (SOC) which opens an energy gap  in the bulk and generates conducting edge [in two dimensions (2D)] or surface [in three-dimensions (3D)] electron states robust against backscattering off nonmagnetic disorder. Interestingly,  Bi2Te3 as one of the prime examples of 3D TIs} is well-known to be one of the best bulk thermoelectrics with ZT ~ 1. Recent efforts have also demonstrated how using nanocomposites of bulk and thin film  Bi2Te3  can lead to ZT ~ 2.5. However, none of these findings relies on the topological surface states whose contribution to electric conductance and the Seebeck coefficient would be insensitive to disorder introduced to suppress lattice thermal conductance. 

Using combined first-principles and quantum transport calculations, the UD and Japanese team has designed in silico a nanoscale thermoelectric based on graphene nanoribbons (GNRs) with heavy adatoms and nanopores. The adatoms locally enhance SOC in graphene to convert it into a 2D TI with a band gap in the bulk and robust helical edge states, which carry electrical current and generate a highly optimized power factor S2G per helical conducting channel. Concurrently, the array of nanopores impedes the lattice thermal conduction through the bulk. The thermoelectric figure of merit is found to reach its maximum ZT ~ 3 at low temperatures T ~ 40 K, which could be attractive for applications in radioisotope thermoelectric generators on spacecrafts or cooling of electronic satellite components. While bulk materials are deemed necessary for large-scale power generation, GNRs underlying this design are single-atom-thick and with electronic transport properties which do not scale with their width, so that very high packing density of GNRs connected in parallel is possible within a 3D volume.

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Nano Letters published by American Chemical Society reports on fundamental research in all branches of the theory and practice of nanoscience and nanotechnology, providing rapid disclosure of the key elements of a study, publishing preliminary, experimental, and theoretical results on the physical, chemical, and biological phenomena, along with processes and applications of structures within the nanoscale range.  Out of 69 journals in Nanoscience and Nanotechnology, Nano Letters is #2 in citations and impact factor (13.025 in 2013).